Crossword Genius

Brittle as iron strongbox (4)

Ross

I believe the answer is:

safe

I'm a little stuck... Click here to teach me more about this clue!

'strongbox' is the definition.
(I know that safe is a type of strongbox)

'brittle as iron' is the wordplay.
'brittle as' becomes 'sa' (I can't explain this - if you can you should give a lot more credence to this answer).
'iron' becomes 'fe' (Fe is the chemical symbol for iron).
'sa'+'fe'='SAFE'

Can you help me to learn more?

(Other definitions for safe that I've seen before include "Secure and protected" , "Metal case for storing valuables" , "Secure in metal box?" , "Secure and free from danger" , "Uninjured, secure" .)

I've seen this clue in The Mirror.
Want a hint initially instead of a full solution? Install my app